Monocrystalline powder and monograin membrane production

ABSTRACT

For production of monocrystalline powders there is formed a melt to which a fluxing agent is added. The melt contains the components of a semiconductor material, an example being the components of copper indium diselenide which are generally used in a stoichiometric composition. The melt is usually heated to temperatures of between 300° C. and 1000° C. Monocrystalline powder grains grow. The desired recrystallization takes place at temperatures above the melting points of the materials to be fused. Once the powder grains have the desired size, the growth is stopped by quenching. The appropriate instant of quenching as well as the appropriate temperature profile for obtaining desired powder sizes are determined by, for example, preliminary experiments. Thereafter the fluxing agent is eliminated. Monograin membranes are produced from the powders produced according to the process and are used in particular in solar cells. The process is simple and inexpensive. Powder grains of uniform size are obtained.

CROSS REFERENCE TO RELATED APPLICATION

This application is a U.S. National Phase Application under 35 USC 371of International Application PCT/DE99/01870 (not published in English)filed Jun. 23, 1999.

BACKGROUND OF THE INVENTION Field of the Invention

The invention relates to a process for production of monocrystallinepowders and a monogram membrane comprising the same.

BACKGROUND INFORMATION

A monogram membrane is a thin film constructed from one layer of powder.The powder grains are bonded together.

From the article entitled “Monograin layers” by T. S. Velde and G. W. M.T. van Helden in Philips Technical Review, 29 (1968), 238-242, it isknown that a monogram membrane can be produced from monocrystalline CdSpowder. Monocrystalline powder comprising CdS is obtained by crushing arelatively large single crystal. A bonding agent is then applied as athin film on a glass substrate. The powder is scattered on the film ofthe bonding agent. Thereupon a layer of the powder adheres to thebonding agent. The other powder grains not attached to the bonding agentare eliminated. Dissolved resin, polymer or components for the same areadded to the powder grains adhering to the bonding agent. After thesolution has been dried and cured, the film containing a powder layer ispeeled from the substrate. If necessary, the powder grains can beexposed by etching, starting from the surface. Otherwise the powdergrains are or remain held together by the resin, etc. and thus form thedesired monogram membrane.

One problem is the production of the monocrystalline powder. Forexample, it is relatively expensive first of all to produce a relativelylarge single crystal. It is also hardly possible to produce powdergrains of uniform size by mechanical crushing.

Powder grains of uniform size are necessary in order to obtain amonogram membrane of uniform thickness.

A monogram membrane can be used advantageously in the art ofphotovoltaics, among others. Copper indium diselenide is a particularlysuitable material for this purpose.

SUMMARY OF THE INVENTION

The object of the invention is to provide an inexpensive process forproduction of monocrystalline powder with predetermined grain sizes. Afurther object of the invention is to provide, for the first time,particular monogram membranes comprising powders formed according to theprocess.

The objects are achieved by a process having the features describedhereinbelow as well as by a monogram membrane having the featuresdescribed hereinbelow. Advantageous embodiments are specifiedhereinbelow.

The present invention concerns a process for producing a monocrystallinepowder comprising a semiconductor material. The process comprises:

(a) fusing together individual components of the semiconductor materialor salts thereof to form a melt;

(b) adding a fluxing agent to the melt;

(c) adjusting the temperature of the melt together with the fluxingagent such that the components or salts thereof melt and at the sametime the powder to be produced crystallizes out, so that monocrystallinepowder grains grow; and

(d) cooling the melt such that the growth of the monocrystalline powdergrains is stopped.

The present invention also relates to a monogram membrane comprisingmonocrystalline copper indium diselenide or GaAs grains producedaccording to the above-described process.

DETAILED DESCRIPTION OF THE INVENTION

According to the process, a melt is formed and a fluxing agent is added.The melt is formed from the individual components of a semiconductormaterial, preferably a II/VI or III/V semiconductor, an example of whichcan therefore be the components of copper indium diselenide or GaAs.Salts containing the components can also be fused instead of thecomponents. The components or their salts are preferably chosen suchthat the components are present in the melt in the same stoichiometriccomposition as that of the powder to be produced.

The melt must then be brought to a temperature at which the individualcomponents or their salts become fused and at the same time the powderto be produced crystallizes out. Such a temperature typically liesbetween 300° C. and 1000° C. In the appropriate temperature range,monocrystalline powder grains are formed in the melt. Once the powdergrains have reached the desired size, the melt is cooled or quenched sorapidly that the growth of the powder grains is stopped as a result. Theappropriate instant of quenching, as well as the appropriate temperatureprofile for obtaining desired powder sizes are determined by, forexample, preliminary experiments. After quenching or cooling it isexpedient to eliminate the fluxing agent.

The process is simple and inexpensive, since it is not necessary toproduce large single crystals beforehand. The grains grow uniformly, andso the resulting powder comprises grains of uniform size.

To produce copper indium diselenide monocrystalline powder, the saltmelt can be formed from CuSe and In, or from Cu, Se and In, or fromCu—In alloys and Se, or from Cu, In or Se salts with appropriate meltingpoints. A typical melt then has the composition of, for example, 6.35 gCu, 11.5 g In, 15.8 g Se and 40 vol % CuSe.

NaCl or an excess of Se or selenides can be used as a fluxing agent in amelt containing copper indium diselenide. The proportion of fluxingagent typically amounts to 40 vol % of the melt. In general, however, itcan range between 10 vol % and 90 vol %. The melt together with thefluxing agent is introduced into, for example, a quartz ampoule. Thequartz ampoule is evacuated and fused. Thereafter the quartz ampouletogether with the contents cited as an example is heated to at least300° C., especially 600° C. As soon as the components have melted,monocrystalline copper indium diselenide grains begin to grow. Thegrowth of a semiconductor such as copper indium diselenide takes placeas a function of time and of the fluxing agent used. Depending onfluxing agent and desired size of the powder grains, a treatment timeranging from 5 minutes to 100 hours is necessary.

In order to stop the growth selectively, the melt is cooled. The coolingrate determines the fault content and fault type in the material, aswell as the surface morphology. Quenching can be completed within a fewseconds. The melt can also be cooled over a period of several hours. Forthis purpose the quartz ampoule together with the contents can be cooledin a water bath or in air at an instant determined by preliminaryexperiments. Thereafter the contents are removed from the quartz ampouleand the fluxing agent is eliminated. In the case of NaCl, this can beachieved, for example, by dissolving the NaCl in water, provided thepowder grains are insoluble in water, as is the case of copper indiumdiselenide. If Se is used as the fluxing agent, it can be eliminated byvolatilization of Se.

The temperature range in which recrystallization takes place depends onthe fluxing agent and the desired grain size, and can lie between 100°C. and 1000° C. The process has been used to produce, among othersubstances, monocrystalline copper indium diselenide powder withextremely high electrical conductivity. Grain diameters of 40 μm, forexample, have been obtained. Grains with resistance of 10 to 30Ω havebeen achieved. These values correspond to specific electricalresistivities of 0.1 to 0.6 Ωm.

It was possible to produce powders with diameters of 0.1 μm to 0.1 mm.

From the powders produced according to the process, there can beproduced by the prior art described hereinabove in the background of theinvention monogram membranes which can be used, for example, inphotovoltaics. A minimum diameter of 10 μm was necessary for productionof monogram membranes, since otherwise a continuous polymer film was notpossible. A diameter of 50 μm should not be exceeded for the productionof monogram membranes, since otherwise, in the art of photovoltaics, forexample, undesirably high series resistances develop and material iswasted. It is worth emphasizing that the grain sizes produced accordingto the process vary only slightly within a batch.

Further examples of semiconductor materials from which monocrystallinepowders can be produced according to the process are CdTe, CdSeTe, CdS,CdSSeTe, GaAs, InP.

What is claimed is:
 1. A process for production of a monocrystallinepowder comprising a semiconductor material, the process comprising: (a)fusing together individual components of the semiconductor material orsalts of the components to form a melt, (b) adding a fluxing agent tothe melt, (c) adjusting the temperature of the melt together with thefluxing agent contained therein such that the components or their saltsmelt and at the same time the powder to be produced crystallizes out, sothat monocrystalline powder grains grow, and (d) cooling the melt byrapidly quenching the melt such that the growth of the monocrystallinepowder grains is stopped.
 2. The process according to claim 1, whereinthe fluxing agent is eliminated after the cooling.
 3. The processaccording to claim 1, wherein the fluxing agent is selected from thegroup consisting of NaCl, Se, As, an arsenide and a selenide.
 4. Theprocess according to claim 1, wherein the fluxing agent is contained inthe melt in a proportion of 10 vol % to 90 vol %.
 5. The processaccording to claim 1, wherein the semiconductor material comprises aGroup II/VI semiconductor or a Group III/V semiconductor.
 6. The processaccording to claim 2, wherein the fluxing agent is selected from thegroup consisting of NaCl, Se, As, an arsenide and a selenide.
 7. Theprocess according to claim 2, wherein the fluxing agent is contained inthe melt in a proportion of 10 to 90 vol %.
 8. The process according toclaim 3, wherein the fluxing agent is contained in the melt in aproportion of 10 to 90 vol %.
 9. The process according to claim 6,wherein the fluxing agent is contained in the melt in a proportion of 10to 90 vol %.
 10. The process according to claim 2, wherein thesemiconductor material comprises a Group II/VI semiconductor or a GroupIII/V semiconductor.
 11. The process according to claim 2, wherein thesemiconductor material comprises copper indium diselenide or GaAs. 12.The process according to claim 11, wherein in step (b), the temperatureis 300° C. to 1000° C.
 13. The process according to claim 1, wherein thesemiconductor material is selected from the group consisting of CdTe,CdSeTe, CdS, CdSSeTe, GaAs, InP and CuSeIn.
 14. The process of claim 1,wherein the quenching is carried out in a few seconds.